It's unable to read a file on the filesystem so it's failing. You may have corrupted files on the filesystem, incorrect permissions/ownership on the files/folders, etc. There is no way for us to tell unless you look deeper into this. All we can see is the system cannot read the file /path/to/attachments/b/b56944cb4722cc5cda9d1e23a3ea7fbc
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Cheers.